The subject of the invention is a method employed in long-hole drilling in which a bit which drills a full hole is used and to which percussive energy is transferred through a steel drive rod. The rod is made up of joined extension pieces according to the depth of the hole and which are joined one to another by means of a threading system. The extension piece has an internal thread at one end and on the other end, a corresponding external thread so that the extension pieces externally form a continuous, stepless drill rod. Also in continuation of the drilling rod, there is provision in its axial direction for an internal hole or central flushing channel so that, according to the invention, intermediate pressurized fluid flushing media such as water or air can be brought into the hole to be drilled and with the aid of which the particles are conveyed out of the hole through an annular space or side flushing channel between the outer surface of the drill rod and the wall of the drilled bore. In this method the diameter of the bore to be drilled varies between 30 mm and 300 mm and the drilling machine proper is arranged so as to operate outside the hole to be drilled.
In rock drilling, a pneumatic drill, that is a drill operating on compressed air, is used. In a pneumatic drill, the greater part of the energy is used percussively although a considerable part of it is also consumed by the flushing action. In recent times, hydraulic drilling machines have, however, come onto the market and in these the energy used for percussion is fundamentally less than in pneumatic drilling machines. The development of fully hydraulic drilling machines has made it possible to economically produce high striking efficiency, so that the drilling of larger holes has become possible. Because in the hydraulic drilling machine, relatively less percussive energy is used, the significance of the energy consumed in flushing has become more important. Thus a drawback which has long been experienced in drilling has become emphasized. This drawback is the low efficiency ratio of the flushing system, occurring especially in the drilling of larger holes in excess of 75 mm diameter. When drilling large holes with the drill rods used today an exceptional quantity of flushing air is needed. Thus the situation arises in which the flushing uses even more energy than percussion. This is caused by the fact that with conventional drill rods, it is not possible to achieve sufficient flushing action to correspond with the increased drilling efficiency.
The drill rod has two functions in percussive rock drilling. First, it transfers the striking, rotation and feeding forces from the drilling head to the drilling bit. Secondly, it acts as part of a conveying system that removes the loosened rock material from the drilled hole. This transfer is carried out by a flushing system, which consists of the drill rod flushing hole or central flushing channel, through which the flushing substance is led to the bottom of the drilled hole. The drill bit connected to the lower part of the drill rod directs the flow of the flushing substance to the bottom of the drilled hole so that the hole is cleaned as efficiently as possible. The flushing substance and loosened rock material are led out of the hole through the side flushing channel, which is formed by the space between the hole which has been drilled and the drill rod.
In order that the loosened rock material may be carried away efficiently enough, the flow velocity of the flushing air or water between the hole to be drilled and the drill rod must be sufficiently high. When using air flushing, the velocity should be at least 25 m/s and with water flushing 1 m/s. If the flow rate of the intermediate flushing substance is not high enough, the drill bit has to crush the rock material several times before it is fine enough to be carried out of the hole by the flushing substance. The markedly increased efficiency of hydraulic drilling machines has led to this situation. When the flushing efficiency cannot correspond efficiently enough to the drilling efficiency, it diminishes the penetration force of the drill, thus also dimishing the efficiency of drilling.
Drill rod dimensions are determined in two stages according to an established method. Firstly, the drill rod diameter is determined in order that it can withstand percussive rotational and feeding forces imposed on it by the drilling machine. Secondly the size of the flushing hole is selected. When that portion of the area of the hole to be drilled has been allocated to the drill rod, sufficient for the mechanical forces it must transmit, the rest is to be used as efficiently as possible for removal of the loosened rock material from the hole.
The size of the flushing hole in the drill rod is normally 7-15% of the area derived from the outside diameter of the rod. A larger flushing hole is particularly needed in downwardly-fed long-hole drilling.
The optimum design dimensions for a flushing system should meet the two following conditions. Sufficient flow velocity of the flushing substance should be attainable in the side wall channel with the least possible pressure and flushing medium quantity, that is with the corresponding least possible energy consumption. The side flushing channel must also be sufficiently large so that the loosened rock material can pass through even when drilling with a worn drill bit. The first of the conditions is fulfilled when the flow resistance of the flushing system is at a minimum. For fulfillment of the second condition, the difference between the drill rod must be at least 10-15 mm. Then, even the largest loosened rock fragments are removed without impediment from the hole.
Since the size of the flushing hole in a drill rod (the central flushing duct) in accordance with modern practice is normally 7-15% of the rod area given by its outside diameter, the proportion of the surface area of the hole to be drilled corresponding to the central flushing hole is only 2-7%. In drilling the rather larger holes, the portion between the drill rod and the hole to be drilled, that is the flushing channel cross-sectional area, is many times greater in comparison with that of the central flushing duct. This means that with the commonly used conventional drill rod, the central flushing duct constitutes a throttling section in the flushing system which is an ample source of energy consumption, and through which it is not possible to lead a sufficient quantity of flushing substance. When on the other hand, the side flushing channel is large in comparison with the central flushing duct, the flow rate of flushing substance in the side flushing channel cannot be made sufficiently high to lift the loosened rock particles out of the drilled hole.